Patients with poor atrio-ventricular conduction or poor sinus node function typically receive pacemaker implants to restore a normal heart rate. For another set of patients suffering from left bundle branch block (LBBB), left ventricular pacing and/or bi-ventricular pacing has been shown to significantly improve cardiac hemodynamics and quality of life. However, some studies have shown that traditional pacing from a right ventricular (RV) apex can impair cardiac pumping performance. In some instances, ventricular wall abnormalities (ventricular remodeling) resulting from RV apical pacing have also been observed. So, alternative sites have been found where pacing can cause an electrical activation sequence similar to that in a normally activated heart and thus contribute to improved cardiac pump function.
From the literature there appear to be three major characteristics of normal cardiac electrical activation: 1. ) Earlier activation of the left ventricle than right ventricle; 2. ) Earlier endocardial activation than epicardial activation in left ventricular free wall; and 3. ) Earlier activation in the apex than in the base of both ventricles. It has been found that a site of earliest activation occurs in the endocardium of the left ventricle along a lower portion of the inter-ventricular septum (i.e. near the apex) where the septum joins with the anterior wall of the heart and in close proximity to bases of left ventricular papillary muscles. Proper timing of papillary muscle activation is necessary to cause closure of the mitral valve prior to main left ventricular contraction so that regurgitation is prevented.